I have experienced the same thing. Even with a real cock anything beyond 6” causes pain. I choose a position that limits how far a guy can go in.
In a good or a bad way. My friend who takes long ones loves the depth but won't let me ass fuck her. Too thick and it hurt her once so no more. But she loves things in her bottom and I can't remember the last time we didn't use at least two toys in her backside.
I enjoy the feeling of an 8” dildo in my ass. My wife only likes fingers or my cock, which I suppose is about 5.5-6”. She finds dildos too rigid for her asshole, and doesn’t have any interest in a butt plug.
With my dildo I can only get about six inches in. I've taken longer real cocks but it gets very uncomfortable after a while. It did make me cum hands free. Something with the gerth pressure and pain pushes me over the top.